Two things to try, 1st: Memory - Set Maximum Amount Used by Windows 7
1) Start> In search box type msconfig.exe Click on msconfig.exe
2) On System Configuration dialog box select the Boot tab
3) Click Advanced Options
4) Uncheck the Maximum Memory option
5) OK your way out
If that doesn't fix it, sometimes rearranging the physical Ram sticks can fix this. Move them to different slots.